Chopper stabilized operational amplifiers have been used to reduce both DC offsets and low frequency flicker noise. These amplifiers, traditionally, have the undesirable property of having a large tone at the chopper frequency. Systems to reduce these effects include various forms of filtering. An example of a conventional chopper stabilized operation amplifier 100, using a filter, can be seen in FIG. 1. Amplifier 100 is generally comprised of an input stage 116, amplifiers 106, 112, and 114, notch filter 110, and capacitors C1 through C4. The input stage 116 includes a chopper or mixer 102 and 108 at the input and output of amplifier 104, respectively.
In operation, the mixers 102 and 108 are driven by a clock signal Φ1 to effectively flip the differential signal INM and INP at the input and output substantially simultaneously. From an input signal standpoint, the signal INP and INM does not change polarity with the exception of a small glitch period during the switching of the signal INP and INM. The offset and flicker noise in input stage 116, however, is only flipped at the output, since the offset occurs after the mixer 102. Thus, the offset toggles as the input and the output current from input stage 116 toggles. If the amplifier 104 is coupled through feedback, the net result is the input has a signal that is increased or decreased by the input referred offset. The average of this toggling is zero, and the flicker noise and offset are cancelled.
One drawback, however, is that a ripple tone (due to the toggling) is produced. As this ripple tone is undesirable in some applications, it is filtered. The synchronous notch filter 110 is employed to perform this filtering by using clock signal Φ2, which is orthogonal to clock signal Φ1. Since the input is “toggling” at the offset level (without the notch filter 110) the output current from input stage 116 is also toggling. This output current is integrated to produce the triangle waveform as shown in FIG. 2. Here, the notch filter 110 essentially integrates the upper and lower triangles and sums the values of the integrations (which is then amplified by amplifier 112). Additionally, a feed forward nested miller loop is employed (using amplifiers 106 and 114) to compensate for the delay generated by the notch filter 110.
Some other conventional circuit are: U.S. Pat. No. 7,292,095; U.S. Pat. No. 7,446,606; Burt et al., “A Micropower Chopper-Stabilized Operational Amplifier Using a SC Notch Filter With Synchronous Integration Inside the Continuous-Time Signal Path,” IEEE Journal of Solid-State Circuits, Vol. 41, No. 12, pp. 2729-2736, December 2006.